The energy loss of medium-energy He+ ions backscattered from a Cu(100) surface

Abstract

A model is presented for the shape of the surface peak in the energy spectrum of backscattered ions in a channeling and blocking experiment. The elastic energy loss distribution of the ions is calculated by use of Monte Carlo simulation, The inelastic energy loss distribution is calculated by use of data obtained from gas phase ion-atom collisions. The model for the shape of the surface peak is applied to the analysis of energy spectra of 175 keV He + ions backscattered from a Cu(100) surface. It is found that, under channeling and blocking conditions, the inelastic energy loss in surface layers can be almost three times higher thar. .mder normal (i.e. random-incidence and -detection) conditions. It is therefore concluded that the inelastic energy loss depends strongly on the impact parameter of the collision. In~~uctionSlowing down of fast ions in solids has been a subject of study for many decades. The energy loss by elastic processes can in principle be calculated precisely. Its relative contribution to the stopping power (= average energy loss per unit of path length) is, however, almost negligible if the kinetic energy of, for example, He ions is above 25 keV. The inelastic processes, on the other hand, still lack a complete quantitative theoretical description. The model of Lindhard [3] (free-electron gas model) generally predicts quite well the electronic stopping power in terms of energy transfer from a charged particle to a plasma of free electrons. In the local-density approximation of the free-electron gas model of Lindhard [4], the (local) electronic loss is assumed to be a function of the local electron density and of the velocity and charge of the ion only. The stopping power is then obtained by an integration of the local energy loss over the volume of one atom in the solid. In 1967 Bonderup [SJ applied the free-electron gas model in the local-density approximation and the Lenz-Jensen atom model to evaluate the electronic energy loss of protons in a solid. Ziegler [6] has used Hartree-Fock electron density functions in his comprehensive summary of the stopping power of ions in matter.Under many different circumstances, however, deviations between the energy loss per unit of path length and the stopping power are observed or, at least, expected. Experiments about the stopping of fast ions in single-crystals indicate that the energy loss depends on the ion trajectory. For example, much attention has been given to the energy loss under planar or axial channeling conditions. Under these conditions ions will, on the average, not traverse the entire volume of the stopping medium with equal probability. Channeled ions traverse regions with low electron density in the midst of the channels and regions with high density near the walls. Therefore, the energy loss might deviate from the stopping power. Measured ratios for the energy loss in channeling directions and in random directions vary from almost unity [7] to a factor of 4 [8,9]. Trajectory dependence of the energy loss has been observed, for examp...